1. Field of the Invention
The present invention relates to a method for measuring the nonlinear optical properties of various types of optical fibers to be used for optical communications, and an optical amplifier and an optical transmission system, each of which uses the method of measuring the nonlinear optical properties to control an optical input level to an optical fiber, thereby suppressing an occurrence of a nonlinear optical effect.
2. Description of the Related Art
In an optical transmission system, an optical amplifier is used for compensating for a transmission loss in an optical fiber or a loss in an optical function device. In a long-distance trunk system, with an increase of communication demand due to the spread of the Internet, there is introduced a wavelength division multiplexing (WDM) optical transmission system which applies the broadband property of the optical amplifier. Further, a WDM optical transmission system having a wavelength routing function is now being introduced together with the optical amplifier also in metropolitan ring networks.
As typical optical amplifiers, there are cited, for example, a rare-earth element doped optical fiber amplifier, a semiconductor optical amplifier (SOA), an optical fiber Raman amplifier and the like. Further, as rare-earth elements for the rare-earth element doped optical fiber amplifier, there are known erbium (Er) for amplifying a light in 1525-1625 nm wavelength band, thulium (Tm) for amplifying a light in 1480-1510 nm wavelength band, praseodymium (Pr) for amplifying a light in a wavelength band in the vicinity of 1300 nm and the like. At present day, in the optical transmission system, an erbium-doped optical fiber amplifier (EDFA) is mainly used.
Furthermore, the above EDFA is broadly classified into a C-band EDFA of which amplification band is in 1530-1565 nm and an L-band EDFA of which amplification band is in 1570-1605 nm. The L-band EDFA has a feature of having an erbium-doped fiber longer than that of the C-band EDFA.
As described in the above, since the C-band EDFA as well as the L-band EDFA has the amplification band of about 35 nm, for example if a plurality of signal lights contained in a WDM light is arranged at 0.8 nm (about 100 GHz) spacing, it becomes possible to collectively amplify the signal lights of 40 or more waves. Moreover, the EDFA is excellent in view of high output power, and therefore, for example the optical output power of 200 mW or more can be easily obtained. Using such characteristics, the EDFA is applied to various WDM optical transmission systems.
FIG. 7 is a diagram showing an example of use of an optical amplifier in a typical WDM optical transmission system. In this WDM optical transmission system, optical signals of different wavelengths respectively output from a plurality of electric/optical (E/O) converters 111 are multiplexed by a wavelength multiplexer 112 to be input to a post-amplifier 113. In the post-amplifier 113, the input WDM light is collectively amplified to have a predetermined gain or an optical output power level, to be sent out to a transmission fiber 100. Then, the WDM light, which has been propagated through the transmission fiber 100 to be attenuated, is again amplified by a pre-amplifier 121 to have the predetermined gain or the optical output power level. The WDM light output from the pre-amplifier 121 is demultiplexed by a wavelength demultiplexer 122, and the optical signals of respective wavelengths are input to respective optical/electric (O/E) converters 123. Further, for the pre-amplifier 121, there is typically used a configuration in which a dispersion-compensating fiber (DCF) 121A for compensating for the chromatic dispersion occurred in the transmission fiber 100 is disposed, and this dispersion-compensating fiber 121A is arranged between optical amplifiers 121B and 121C of two-staged configuration.
In the WDM optical transmission system as described above, in order to improve an optical signal-to-noise (S/N) ratio, it is desirable to increase the power of the transmitted WDM light as high as possible. However, a nonlinear optical effect occurred in the transmission fiber 100 or in the dispersion-compensating fiber 121A depends on the power of the light propagated through the optical fiber, and therefore, causes the noise or the waveform distortion to degrade transmission characteristics of the WDM light. Accordingly, an upper limit of an optical input level to the optical fiber is generally restricted by the nonlinear optical effect. As the nonlinear optical effects which degrade the above transmission characteristics, there are cited the self-phase modulation (SPM), the cross phase modulation (XPM), the four-wave mixing (FWM), the stimulated Raman scattering (SRS) and the like.
FIG. 8 shows examples of optical input levels at which the nonlinear optical effects occur in various types of optical fibers (to be referred to as “occurrence level of the nonlinear optical effect” hereunder). Incidentally, herein, the assumption is made on the optical transmission system corresponding to the WDM light of C-band and of 10 Gb/s.
It is known that the occurrence level of the nonlinear optical effect is determined according to an optical transmission system model (for example, the wavelength band, the number of wavelengths, the wavelength spacing, a transmission distance per one span, the number of spans and the like), and also, according to types of optical fibers used for the transmission path, the dispersion compensator and the like (for example, a single mode fiber (SMF), a dispersion-shifted fiber (DSF), a non-zero dispersion-shifted fiber (NZ-DSF), a dispersion-compensating fiber (DCF) and the like) and fiber parameters (for example, an effective core cross sectional area, the chromatic dispersion, the effective fiber length and the like). As shown in the examples of FIG. 8, the optical input level to the optical fiber is generally restricted by the self-phase modulation (SPM) except for the dispersion-shifted fiber in which the chromatic dispersion in 1550 nm is 0 ps/nm/km.
In order to avoid the occurrence of the nonlinear optical effect as described above, it is necessary to restrict the optical input level to the optical fiber to be used for the transmission path and the like, that is, the optical output power level of the optical amplifier. However, in an actual operation, there are variations in fiber parameters, a connector loss, a splicing loss and the like, resulting in a problem in that an occurrence level of the nonlinear optical effect cannot be exactly grasped. In order to cope with such a problem, how the nonlinear optical properties of the optical fiber to be actually used are accurately measured to reflect the measurement result on a control of the optical amplifier becomes a major issue.
As a conventional technology related to the measurement of the nonlinear optical properties, there has been known a measuring method as disclosed in “An optical measuring device for user engineers”, edited by Toshiharu TAKOU, Tatsuatsu HONDA, enlarged and revised edition, 1998, pp. 102-126. Hereunder, the description will be made on the summary of the conventional measuring method of the nonlinear optical properties.
For example, the nonlinear refractive index of a silica-based optical fiber is about 2.2×10−20 m2/W. This nonlinear refractive index is relatively small compared with those of other nonlinear mediums. However, since a mold diameter thereof, which is a feature of optical fiber, is small (for example, about 10 μm) and a loss thereof is significantly small, (for example, about 0.2 dB/km in 1.55 μm), it is possible to observe nonlinear optical phenomena. Most of nonlinear optical phenomena occurred in the optical fiber are caused by the nonlinear refraction. These nonlinear optical phenomena occur because the refractive index n of the optical fiber depends on the intensity P of a light as shown in the following formula (1).n=nL+n2P  (1)In the above formula, nL is the linear refractive index of the optical fiber, n2 is the nonlinear refractive index depending on a material for the optical fiber, and P is the input power to the optical fiber.
If the refractive index depends on the optical intensity, there occur some nonlinear optical phenomena. Most widely studied ones from among the nonlinear optical phenomena are the self phase modulation (SPM) and the cross phase modulation (XPM).
The SPM means the phase shift which is caused by the light itself when the light is being propagated through the optical fiber. A phase shift amount φ thereof is expressed by the following formula (2).
                    ϕ        =                                            2              ⁢              π                        λ                    ⁢                      L            ⁡                          (                                                n                  L                                +                                                      n                    2                                    ⁢                  P                                            )                                                          (        2        )            
In the above formula (2), the term depending on the optical intensity corresponds to a phase change due to the SPM, and if this term is φNL, φNL is expressed by the following formula (3).
                              ϕ          ⁢                                          ⁢          NL                =                                            2              ⁢              π                        λ                    ⁢          L          ⁢                                          ⁢                      n            2                    ⁢          P                                    (        3        )            
In the above formula (3), L is the fiber length. However, if a proportional constant of the optical fiber in 1.55 μm is α considering the loss in the optical fiber, the effective fiber length Leff is expressed by the following formula (4).
                              L          eff                =                              (                          1              -                              ⅇ                                                      -                    α                                    ⁢                                                                          ⁢                  L                                                      )                    α                                    (        4        )            
Further, since the optical intensity in the optical fiber is distributed in a core direction of the optical fiber, it is necessary to define an effective core cross sectional area Aeff for a core of the optical fiber. The effective core cross sectional area Aeff can be approximated in accordance with the following formula (5), as a function of a mode field diameter MFD of the optical fiber.
                              A          eff                =                              π            ⁡                          (                              MFD                2                            )                                2                                    (        5        )            
Based on the above formulas (3), (4) and (5), a nonlinear phase change amount φNLSPM due to the SPM can be expressed by the following formula (6).
                              ϕ          NL          SPM                =                                            2              ⁢              π                        λ                    ⁢                                    L              eff                                      A              eff                                ⁢                                    n              2                        ·            P                                              (        6        )            
If the SPM is generated, a wavelength of an input light is spread and at the same time, an optical spectrum waveform thereof is also changed. One example of the conventional SPM measuring system utilizing this phenomenon is shown in FIG. 9. In this measuring system, an output light from a light source (DFB-LD) 131 is amplified by an EDFA 132, and a light from which ASE is eliminated by an optical band-pass filter 133 is input to a measuring objective optical fiber 135 via an optical attenuator 134. The optical attenuator 134 is for varying the intensity of the input light to the optical fiber 135. An output light from the optical fiber 135 passes through a 0.1 nm narrow-band optical filter 136, so that the optical intensity thereof is observed by an optical power meter 137. The measurement of the SPM is performed such that the optical power near the center of optical spectrum of which band is restricted by the narrow-band optical filter 136 is detected while the intensity of an input light pulse being changed, and a change in the optical power near the center to the peak intensity of the input light pulse is measured.
The XPM means the nonlinear phase shift which occurs in one of lights of different wavelengths when the lights of different wavelengths are simultaneously propagated in a same direction. If the optical intensity for different wavelengths λ1 and λ2 are P1 and P2, the nonlinear phase shift to λ1 is expressed by the following formula (7).
                              ϕ          NL                =                                            2              ⁢              π                                      λ              1                                ⁢                                    L              eff                                      A              eff                                ⁢                                    n              2                        ·                          (                                                P                  1                                +                                  2                  ⁢                                      bP                    2                                                              )                                                          (        7        )            
Two terms on the right-hand side in the formula (7) are due to the SPM and the XPM. Accordingly, the nonlinear phase change amount φNLXPM due to only the XPM is expressed by the following formula (8).
                              ϕ          NL          XPM                =                                            4              ⁢              π                                      λ              1                                ⁢                                    L              eff                                      A              eff                                ⁢                      n            2                    ⁢                      bP            2                                              (        8        )            In the above formula, b is a coefficient depending on polarization states of the wavelengths λ1 and λ2, and in the case where the input light is not polarized, b has a value of b=⅔.
One example of a conventional XPM measuring system is shown in FIG. 10. In this measuring system, a pumping light output from a pumping light source 142 which is intensity modulated in accordance with an output signal from an oscillator 141, is depolarized by a depolarizer 143, and this pumping light and a probe light output from a probe light source 144 are coupled by an optical coupler 145 to be supplied to a measuring objective fiber 146, so that the probe light is subjected to the phase modulation through the XPM. A frequency component generated by this phase modulation is received by a self-delayed heterodyne reception system 147, so that a phase shift amount of the probe light can be obtained.
Each of the SPM and the XPM described above is an elastic nonlinear optical phenomenon in which energy is not reciprocated between an electro-magnetic field and the optical fiber. Contrary to this, the stimulated Raman scattering (SRS) and the stimulated Brillouin scattering (SBS) are called stimulated inelastic scattering, since an optical energy as the electro-magnetic field excites an oscillation mode of silica glass to move to the optical fiber medium. A main difference between the SRS and the SBS is in that phonons in optical mode contribute in the SRS, whereas phonons in acoustic mode contribute in the SBS. Each of the SRS and the SBS is a phenomenon in which, when a light of high energy is incident on the optical fiber, a light of different wavelength (Stokes wave) is generated on a longer wavelength (low energy) side of the incident light. Due to a difference between the optical mode and the acoustic mode, the Stokes wave is generated mainly in a forward direction in the SRS, while being generated only in a backward direction in the SBS. A Raman gain spectrum in the silica-based fiber is very broad at about 30 THz, and a frequency shift amount of the Stokes wave is approximately 13 THz. Contrary to this, the Brillouin gain spectral width is very narrow at about 10 MHz, and the frequency shift amount of the Stokes wave is approximately 10 GHz which is smaller than that in the SRS.
On feature common to the SRS and the SBS is in that the SRS and the SBS show the behaviors as if they have thresholds. Namely, only when the optical intensity exceeds a certain threshold, energy conversion into the Stokes wave becomes significant. A threshold PSRSth of the SRS occurrence is expressed by the following formula (9).
                              P          SRS          th                =                  16          ⁢                                                    A                eff                                            L                eff                                      ·                          1                              g                R                                                                        (        9        )            
In the above formula (9), gR is a Raman gain coefficient and has a value of gR≅1×10−13 m/W in the case of the silica-based fiber. Further, a constant 16 on the right-hand side is a value in the case of forward pumping, and has a value of 20 in the case of backward pumping. A pumping wavelength for obtaining the Stokes wave of 1.55 μm band is 1.45 μm band taking the frequency shift of 13 THz into consideration. The effective fiber length Leff is Leff=13 km in accordance with the relation of the above formula (4), provided that, for example, the optical fiber length L=20 km, and α=0.2 dB/km in 1.45 μm band and 1.55 μm band. The effective core cross sectional area Aeff is Aeff=20 μm2, provided that, for example, the mode field diameter of the optical fiber is MFD=5 μm. The threshold PSRSth of the SRS occurrence in this case is 240 mW in the forward pumping while being 300 mW in the backward pumping, and accordingly, the significantly high optical intensity is needed.
Further, a threshold PSBSth of the SBS occurrence is expressed by the following formula (10), similarly to the above formula (9).
                              P          SBS          th                =                  21          ⁢                                                    A                eff                                            L                eff                                      ·                          1                              g                B                                                                        (        10        )            
In the above formula (10), a constant 21 on the right-hand side is a value determined according to the line width of a Brillouin gain, which is an approximation. Further, gB is a Brillouin gain coefficient and has a value of gB≅5×10−11 m/W in the case of the silica-based fiber. The (1/gB) item is smaller than (1/gR) in the case of the SRS by two digits. The threshold PSBSth of the SBS occurrence is 0.6 mW for when the effective fiber length Leff=13 km and the effective core cross sectional area Aeff=20 μm2, so that the SBS can be observed as a scattered light in the backward direction with the optical intensity lower than that in the SRS.
Further, the above formula (10) can be expressed by the following formula (11) taking the line width Δνs of the light source and the Brillouin gain line width Δνb into consideration.
                              P          SBS          th                =                  21          ⁢                                                    A                eff                                            L                eff                                      ·                          1                              g                B                                      ·                                                            Δ                  ⁢                                                                          ⁢                                      v                    b                                                  +                                  Δ                  ⁢                                                                          ⁢                                      v                    s                                                                              Δ                ⁢                                                                  ⁢                                  v                  b                                                                                        (        11        )            In the above formula, the line width Δνb of the light source is about several MHz in the DFB-LD typically used as a light source in the WDM optical transmission system. Further, the Brillouin gain line width Δνb has a value of about 100 MHz in a wavelength of 1.55 μm.
However, in the above described conventional measuring technology of the nonlinear optical properties, the significantly complicated measuring system as shown in FIG. 9 or FIG. 10 is needed. If such a measuring system is incorporated into a WDM optical transmission system using an optical amplifier as shown in FIG. 7 so as to grasp an occurrence state of the nonlinear optical effect in a transmission fiber or a dispersion-compensating fiber, and according to the occurrence state, the optical amplifier is controlled, thereby trying to suppress the degradation of the optical S/N ratio due to the nonlinear optical effect, there is caused a problem in that a configuration of the overall system becomes complicated to lead the rise of cost.